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  • 1
    Electronic Resource
    Electronic Resource
    Fixed angle reactor model calculations for the D+H2(v=0,1)→HD(v'=0,1,2)+H reaction (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 888-898 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An approximate three-dimensional quantum mechanical method, the fixed angle reactor model (FARM), is reviewed and applied to the D+H2→HD+H reaction. Calculated integral and differential cross sections and rate constants are presented and compared with other calculations and with experimental data. Good agreement is found between calculated and experimental rate constants. The effects of tunneling are clearly apparent in the temperature dependence of the rate constant for the D+H2(v=1)→HD+H reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456114
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  • 2
    Electronic Resource
    Electronic Resource
    Hexamethylbenzene as a sensitive nuclear magnetic resonance probe for studying organic crystals and glasses (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6858-6866 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Deuterated hexamethylbenzene (HMB) is used as a probe molecule for 2H NMR studies of the crystalline state of hexachlorobenzene and of several organic glasses. By measuring the spin–lattice relaxation and the line shape in the temperature range of 4–300 K the dynamical parameters of the molecular reorientation are investigated. For the system HMB/hexachlorobenzene, we find exponential relaxation and for the corresponding T1 an increase of its activation energy by a factor of 2 in comparison to the neat HMB. A homogeneous mixing of the guest and host molecules is found at least for guest concentrations up to 7%. In contrast, nonexponential spin–lattice relaxation is characteristic for all glass matrices, indicating motional heterogeneities. A log–Gauss distribution for the corresponding motional correlation times gives a good fit of the data. Its width parameter decreases linearly with temperature, while the mean correlation times are described by an Arrhenius law. The mean activation energy is reduced by a factor of about 3.5 as compared to neat HMB, demonstrating a loose packing of the molecules in the glass matrices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456259
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  • 3
    Electronic Resource
    Electronic Resource
    Vibron band structure in chlorinated benzene crystals: Lattice dynamics calculations and Raman spectra of 1,2,4,5-tetrachlorobenzene (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4023-4034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: From a combined theoretical and experimental study of 1,2,4,5-tetrachlorobenzene (TCB) we conclude that this crystal shows many interesting effects. In agreement with earlier optical measurements, which probe the complete vibron band structure of TCB, we calculate that several of the vibron modes have the dispersion of a one-dimensional crystal with stacks of molecules along the a axis. The inclusion of fractional atomic charges in the atom–atom potential used in the calculations is absolutely necessary to obtain the correct vibron bandwidths. Also the sign of the vibrational coupling matrix elements, which is given correctly by the calculations, is determined by these charges. For other properties, such as the crystal stability, the phonon frequencies, the site splitting in the vibron bands, and also the dispersion of the lower frequency vibrations, the interactions between the "one-dimensional'' stacks are essential, however. The calculations predict further, in qualitative agreement with the Raman spectra, that the splitting between the pairs of inequivalent vibron bands in α-TCB (the site splitting) is significantly larger than the factor group splitting in β-TCB. Finally we have found that, for those vibrations where the conditions are shown by the calculations to be favorable, the 35Cl/37Cl isotope effects are clearly visible in the Raman spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454835
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  • 4
    Electronic Resource
    Electronic Resource
    Magnetic coupling and dynamics in solid α and β-O2. I. An ab initio theoretical approach (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3583-3596 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper describes a new approach to the dynamic and magnetic properties of solid α and β oxygen which is based on two theoretical developments. First, we have constructed the lattice and spin Hamiltonian for solid O2 by including explicitly the interactions between the triplet ground state O2 molecules as obtained mainly from recent ab initio calculations. The spin coupling parameters in this Hamiltonian, especially the Heisenberg exchange parameter J, are strongly dependent on the positions and orientations of the molecules. Secondly, we have developed an integrated scheme for lattice dynamics and spin wave calculations which uses this Hamiltonian. The actual mixing between the lattice modes, phonons and librons, and the magnons appears to be small; their interaction can be largely taken into account by renormalization of the coupling terms. In the lattice dynamics part of the calculation it is essential to include the Heisenberg term, since it is the extremely strong anisotropy of the coupling parameter J that explains the anomalously large libron splitting in α-O2. The spin-wave calculation with the Hamiltonian averaged over the lattice vibrations yields reasonable values for the magnon frequencies with no empirical fit parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451963
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic coupling and dynamics in solid α and β-O2. II. Prediction of magnetic field effects (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 3597-3601 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Via simple thermodynamic arguments and via quantitative lattice dynamics and spin-wave calculations, we predict that the phase transition temperature Tαβ in solid oxygen and the optical libron frequencies in the α and β phases will both be lowered by an external magnetic field. The lowering of Tαβ varies from about 0.1 K at 7.5 T to about 1.2 K at 30 T. The lowering of the Bg and Ag libron peaks in the Raman spectrum of α-O2 and of the Eg peak in the β-O2 spectrum varies between 1.5 and 2.9 cm−1 at 30 T. These shifts can be explained by the magnetic field induced changes in the sublattice magnetizations, which affect the Heisenberg exchange contribution to the intermolecular potential. From ab initio calculations it is known that the Heisenberg coupling parameter J is extremely anisotropic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452737
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  • 6
    Electronic Resource
    Electronic Resource
    Compensating Hamiltonian method for chemical reaction dynamics: Xe desorption from Pd(100) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8444-8453 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on a method that allows us to study with molecular dynamics a chemical reaction with an activation barrier that is much higher than the thermal energy. The method leads to a new Hamiltonian with which it is possible to compute reaction rate constants, and reactive trajectories even at low temperature. The method is applied to Xe desorption from Pd(100). The desorption rate constants are compared with transition state rate constants. The mechanism of the desorption is studied, and a model is given that explains the molecular dynamics results. Conversion of lateral into normal kinetic energy during the desorption process is shown to be important.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460078
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  • 7
    Electronic Resource
    Electronic Resource
    New approach to orientationally disordered molecular crystals (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 1914-1924 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new theory is presented for the description of orientationally disordered molecular crystals. The theory is based on the thermodynamic variation principle with a generalized Ising Hamiltonian. The optimized single-molecule states are calculated, and the occupation of these states and the correlation in the occupation for pairs of molecules is determined via the cluster variation method. The theory is applied to the β phase of solid nitrogen. A delocalized orientational probability distribution is found that is formed by six localized states which are equally occupied. Correlation functions for the orientations of nearest neighbors are given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454115
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  • 8
    Electronic Resource
    Electronic Resource
    A new basis set method for quantum scattering calculations (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6213-6220 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new basis set approach for quantum scattering calculations is described and tested on model problems of elastic and inelastic collisions. The approach is essentially the Kohn variational method, but applied to the S or T matrix directly rather than to the K matrix as is normally done; it is seen that the result of the present approach is not equivalent to the usual Kohn method (i.e., for the K matrix) and is indeed preferable to it. The present approach is seen to have the same structure as the complex scaling/coordinate rotation expressions for the T matrix (but with some added features). Its potential advantage over the Schwinger variational method, another useful basis set technique, is that matrix elements of the Green's function for some reference Hamiltonian are not required; the present method requires only matrix elements of the Hamiltonian itself between the basis functions. The essential reason for all of these desirable features is that the basis set which is used incorporates the correct scattering boundary conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452459
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  • 9
    Electronic Resource
    Electronic Resource
    First-order intermolecular interaction energies from coupled-cluster Brueckner orbitals (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6949-6952 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is investigated to which extent the effect of intramolecular electron correlation on intermolecular first-order Coulomb and exchange energies can be accounted for when solely the reference determinant of the Brueckner coupled-cluster-doubles scheme is used to construct the charge density and the density matrices of the monomers. Considering the dimers He2, Ne2, Ar2, NeAr, NeHF, ArHF, (H2)2, (HF)2, and (H2O)2 the Brueckner orbital approximation in general is found to improve considerably upon the results of the Hartree–Fock determinant. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481309
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  • 10
    Electronic Resource
    Electronic Resource
    Collision effects in the nonlinear Raman response of liquid carbon disulfide (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3277-3285 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model of the polarizability of carbon disulfide dimers was constructed, using polarizabilities from accurate time-dependent density functional theory calculations as reference. This direct reaction field model takes dipole-induced dipole effects, induced multipole effects and effects due to the overlap of the electronic clouds into account in an approximate way. The importance of the induced multipole and the overlap effects is investigated. This polarizability model is subsequently used to calculate the third-order time-domain Raman response of liquid carbon disulfide. These results are compared to experimental data and earlier calculated response in which only dipole-induced dipole effects on the polarizability were included. The multipole effects are found to give a significant contribution to the subpico second part of the third-order Raman response. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1436463
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